1. A new fixing solution is described, which preserves the platelets and prevents contact hemolysis of the erythrocytes, so that counts of both corpuscles may be made in the same preparation. 2. Comparative counts of platelets in arteries and veins show that arterial blood contains a larger number of platelets than venous blood. This difference is accentuated under experimental conditions that cause a reduction in the number of platelets. It is concluded that new platelets are added to the blood in the capillary areas of the lungs, and that there is a corresponding destruction of platelets as the blood passes through the capillary areas of the systemic circulation. 3. Perfusion of the lungs with a platelet-preserving solution, compared with that of other organs, gives evidence of the existence of a source of platelet material in the lungs. 4. Histological examination of the lungs with a technique adequate to give a differential staining of platelet material demonstrates the presence of giant cells in the lungs, and supports the view that they are active in the production of platelets. 5. In extrauterine life giant cells are concentrated in the marrow and the lungs, with the maximum of their activity in platelet production in the lungs.